Simplify the following expression: $p = \dfrac{1}{4t} - \dfrac{7}{9t}$
Explanation: In order to subtract expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $4t$ and $9t$ $\lcm(4t, 9t) = 36t$ $ p = \dfrac{9}{9} \cdot \dfrac{1}{4t} - \dfrac{4}{4} \cdot \dfrac{7}{9t} $ $p = \dfrac{9}{36t} - \dfrac{28}{36t}$ $p = \dfrac{9 -28}{36t}$ $p = \dfrac{-19}{36t}$